Zinnias are the workhorse of the cutting garden — fast to flower, generous with blooms, and almost impossible to kill in full sun. Sow direct into warm soil after last frost and you'll be cutting in eight weeks. The real trick is succession: sow every two weeks through midsummer and you'll still have armfuls when the first frosts arrive.
Pinch the first flower bud to force branching. Every stem you cut — cut all the way back to a leaf node — encourages two more. Zinnias stop producing if you let them go to seed, so deadhead ruthlessly and they'll reward you until the calendar runs out.
Powdery mildew
White or grey powdery coating on leaves — usually starting on older growth in humid conditions or when nights cool.
Improve air circulation by thinning plants. Apply neem oil or potassium bicarbonate spray at first sign. Avoid overhead watering.
Alternaria blight
Brown circular spots with yellow halos on leaves; spreads rapidly in wet weather.
Remove affected leaves immediately. Water at the base only. A copper-based fungicide can help if caught early.